The present invention relates to structured packing and to methods for installing such packing in an exchange column. The structured packing has particular application in cryogenic air separation processes, although it also may be used in other heat and/or mass transfer processes that can utilize structured packing.
The term, “column” (or “exchange column”) as used herein, means a distillation or fractionation column or zone, i.e., a column or zone wherein liquid and vapor phases are counter currently contacted to effect separation of a fluid mixture, such as by contacting of the vapor and liquid phases on packing elements or on a series of vertically-spaced trays or plates mounted within the column.
The term “column section” (or “section”) means a zone in a column filling the column diameter. The top or bottom of a particular section or zone ends at the liquid and vapor distributors respectively.
The term “packing” means solid or hollow bodies of predetermined size, shape, and configuration used as column internals to provide surface area for the liquid to allow mass transfer at the liquid-vapor interface during countercurrent flow of two phases. Two broad classes of packings are “random” and “structured”.
“Random packing” means packing wherein individual members do not have any particular orientation relative to each other or to the column axis. Random packings are small, hollow structures with large surface area per unit volume that are loaded at random into a column.
“Structured packing” means packing wherein individual members have specific orientation relative to each other and to the column axis. Structured packings usually are made of thin metal foil, expanded metal or woven wire screen stacked in layers or as spiral windings.
In processes such as distillation or direct contact cooling, it is advantageous to use structured packing to promote heat and mass transfer between counter-flowing liquid and vapor streams. Structured packing, when compared with random packing or trays, offers the benefits of higher efficiency for heat and mass transfer with lower pressure drop. It also has more predictable performance than random packing.
Cryogenic separation of air is carried out by passing liquid and vapor in countercurrent contact through a distillation column. A vapor phase of the mixture ascends with an ever increasing concentration of the more volatile components (e.g., nitrogen) while a liquid phase of the mixture descends with an ever increasing concentration of the less volatile components (e.g., oxygen). Various packings or trays may be used to bring the liquid and gaseous phases of the mixture into contact to accomplish mass transfer between the phases.
The most commonly used structured packing consists of corrugated sheets of metal or plastic foils (or corrugated mesh cloths) stacked vertically. These foils may have various forms of apertures and/or surface texture features aimed at improving the heat and mass transfer efficiency. An example of this type of structured packing is disclosed in U.S. Pat. No. 4,296,050 (Meier). It also is well-known in the prior art that mesh type packing helps spread liquid efficiently and gives good mass transfer performance, but mesh type packing is much more expensive than most foil type packing.
In conventional practice, corrugated structured packing sheets are substantially uniform in height and have straight cut edges (i.e., “unmodified” edges) such that the base and top of each section or brick are essentially flat. The bricks are stacked one on top of the other to form layers of structured packing.
Multiple layers of structured packing are placed between suitable supports inside an exchange column to form a packed section. Adjacent layers may be rotated relative to each other to facilitate proper spreading and mixing of vapor and liquid during normal operation. Liquid and vapor distributors are placed above and below each packed section to feed such fluids in a uniform fashion into the packed section.
The capacity of structured packing is limited by the resistance to fluid flow at the interfaces between successive layers of packing in a packed section. It is very desirable to increase the capacity of structured packing, since an increase in capacity allows for the use of less structured packing for any given separation, thus reducing the cost of carrying out the separation.
Usually the capacity of structured packing is limited by flooding. Mass transfer flooding, which is the premature degradation in mass transfer performance prior to the onset of hydraulic flooding, occurs when the mass transfer efficiency of the column starts deteriorating rapidly with the increase of vapor and/or liquid flow in the column. Hydraulic flooding occurs when the pressure drop across the packing bed starts increasing rapidly with the increase of vapor and/or liquid flow.
It is known from the prior art that the capacity of structured packing can be increased by modifying the edges of individual packing sheets. Typical modifications include reduced crimp heights, changed corrugation angle, serrations, apertures, etc., which modifications are typically made at the bottom of all sheets or at the top and bottom of alternating sheets. Examples of such modifications are disclosed in U.S. Pat. No. 5,632,934 (Billingham, et al.) and U.S. Pat. No. 6,101,841 (Billlingham, et al.). Other modifications include S-shaped corrugations on both ends of every sheet, such as those disclosed in EP 0 858 366 B1, U.S. Pat. No. 6,206,349 (Parten) and International Application WO 97/16247. All such modifications are made in such a way that during operation the pressure drop in the transitions is reduced. EP 0 858 830 A1teaches that in order to maintain good mass transfer performance it is important to maintain a flat top while making any edge modifications to increase capacity. Operation of a packed column at a pressure drop greater than 0.7 inch water per foot is taught in U.S. Pat. Nos. 5,921,109 (Billingham, et al.) and U.S. Pat. No. 6,212,907 B1 (Billingham, et al). These patents cover cases wherein only the bottoms of the packing sheets are modified, and cases wherein both the tops and bottoms of the packing sheets are modified.
Although the capacity of conventional corrugated structured packing can be increased by modifying the top and/or bottom edges, such edge modified high-capacity packing does not always scale up reliably from laboratory scale small diameter columns to industrial scale large diameter columns in terms of mass transfer performance of the columns. Careful experimentation in small scale laboratory columns can identify geometries that are also suitable for good mass transfer performance. While the increase in capacity scales up reliably, the mass transfer performance can be unpredictable in large scale columns. Adding additional packing height is expensive and can negate the advantage gained from the increase in capacity.
It is desired to have an assembly of structured packing in an exchange column which significantly increases the capacity of the structured packing without any significant degradation in mass transfer performance of the exchange column.
It is further desired to increase the capacity of a section of structured packing in an exchange column by reducing the resistance to fluid flow at the interfaces between layers of packing in the packed section.
It is still further desired to have an assembly of structured packing in an exchange column which provides improved performance over that of conventional structured packing alone.
It is still further desired to have an assembly of structured packing in an exchange column which shows improved performance characteristics for cryogenic applications, such as those used in air separation, and for other heat and/or mass transfer applications.
It is still further desired to have an assembly of structured packing in an exchange column which overcomes many of the difficulties and disadvantages of the prior art to provide better and more advantageous results.
It also is desired to have a method of assembling and installing an assembly of structured packing in an exchange column which affords better performance than the prior art, and which also overcomes many of the difficulties and disadvantages of the prior art to provide better and more advantageous results.